The invention relates to a progressive spectacle lens, the lens including an only slight dynamic change of the as worn properties during horizontal movements of glance.
Progressive spectacle lenses (also called varifocal lenses, multifocal lenses etc.) are usually understood to be spectacle lenses having a different (smaller) power in the region through which a spectacle wearer views an object located at a greater distancexe2x80x94hereunder designated as distance portionxe2x80x94than in the region (near portion) through which the spectacle wearer views a near object. Located between the distance portion and the near portion is the so-called progressive zone in which the power of the spectacle lens continuously increases from that of the distance portion to that of the near portion. The magnitude of the power increase is also designated as the power addition.
As a rule, the distance portion is located in the upper part of the spectacle lens and designed for viewing xe2x80x9cto infinityxe2x80x9d, whilst the near portion is located in the lower region and is particularly designed for reading. With spectacles for special applicationsxe2x80x94those for pilots or for monitor work stations are mentioned as examplesxe2x80x94the distance and near portions also may be arranged differently and/or designed for other distances. Furthermore, it is possible for a plurality of near portions and/or distance portions and corresponding progression zones to be present.
With progressive spectacle lenses having a constant refractive index it is necessary for the curvature of one or both surfaces to change continuously from the distance portion to the near portion, in order that the power may increase between the distance portion and the near portion
The surfaces of spectacle lenses are usually characterized by the so-called principal radii of curvature R1 and R2 at every point of the surface. (Sometimes also the principal curvatures K1=1/R1 and K2=1/R2 are stated instead of the principal radii of curvature). Together with the refractive index n of the glass material, the principal radii of curvature determine the parameters frequently used for an ophthalmologic characterization of a surface:
Surface power D=0.5xc2x7(nxe2x88x921)xc2x7(1/R1+1/R2)
Surface astigmatism A=(nxe2x88x921)xc2x7(1/R1xe2x88x921/R2).
The surface power is the parameter with which an increase of the power from the distance portion to the near portion is achieved. The surface astigmatism (lucidly termed cylinder power) is a xe2x80x9cdisturbing propertyxe2x80x9d, because an astigmatismxe2x80x94inasmuch as an eye does not have an innate astigmatism to be correctedxe2x80x94exceeding a value of about 0.5 dpt results in an indistinct image on the retina being perceived.
Although any change of the curvature of the surface which is needed for achieving a surface power increase without vision being xe2x80x9cdisturbedxe2x80x9d by surface astigmatism can be attained relatively simply along a (plane or curved) meridian, considerable xe2x80x9cintersectionsxe2x80x9d of surfaces will result alongside this meridian, leading to a large surface astigmatism which more or less impairs the lens in regions alongside the mentioned meridian.
A further consequence of these intersections is that the spectacle lens will have different nasal and temporal properties at corresponding see-through positions. Hereby binocular vision, in particular, may be adversely affected.
This becomes apparent mainly during movements of the glance and is thus disturbing:
When a spectacle wearer allows his gaze to pursue a moving object whilst keeping his head at rest, his visual impression will depend, on the one hand, on the imaging quality of the two single lenses of his spectacles at the positions through which he looks whilst making the necessary eye movements, to follow the object. When the spectacle wearer encounters small image defects (astigmatism, refraction defects, etc.) at these see-through positions, he will see the object more distinctly than when large image defects are present.
On the other hand, however, as a rule both eyes participate in seeing, and the total visual impression will be composed of the visual impressions of both eyes.
Thus, it will be of consequence, for example, whether the object can be seen binocularly as being single, how large the effort of fusion is, and whether the spectacle wearer perceives the object equally distinctly with both eyes, or well with one eye and badly with the other eye.
Finally, in the case of moving objects any change of this binocular visual impression during the pursuit of the object is of importance.
The invention is based on an object of further developing a progressive spectacle lens in such manner that the optical parameters relevant to the quality of the image change as little as possible whilst the glance is shifting.
This object is achieved by the invention as described and claimed hereinafter. Further developments of the invention are the subject matter of the dependent claims.
As a rule, spectacle lenses are assessed and described by means of two-dimensional isometric-line representations (plots) of certain properties of the surfaces or the wearing situation. The parameters of interest for the wearing position are obtained, for example, by first establishing the principal ray passing through the point on the spectacle lens and the center of rotation of the eye, and subsequently calculating a wave-front along the principal ray, the properties of which are finally plotted.
With this, the properties peculiar to the local wave-front are obtained for every angle of view.
It is not possible simply to determine from these plots the performance of a spectacle lens during, for example, horizontal movements of the gaze, because the succession of the points of penetration by the principal rays through the spectacle lens caused by the movement is not known.
It would appear obvious to simply read-off the values of a horizontal meridian from the plot, but this is wrong when the vertical prism changes along this meridian, and this is almost always the case.
A typical situation is that of a spectacle wearer being presented with a horizontally moving object point on which his gaze is to be fixed whilst he keeps his head unmoving. Such horizontal movements of gaze frequently occur, for example during reading or when working on a computer. During these movements of the eye the spectacle wearer""s visual impression of the viewed object point will change, because the glance will travel through regions of the lens which have different imaging qualities. The as worn parameters, in particular the surface power and the astigmatism will change.